That's consequential. In step 1 you create a gamut mapping that
compresses the whole, huge ProPhoto space into ISOcoated_v2. In step 3
you use the precalculated gamut mapping of ISOcoated_v2 to compress a
much smaller gamut (AFAIK something like the Pointer space) to
ISOcoated_v2. Step 3 gives you more saturation, but clips everything
outside the Pointer space.